Update 2026-27 Pension Credit guarantee rates

PolicyEngine UK currently appears to use forecast-indexed 2026 Pension Credit guarantee rates instead of the published 2026-27 weekly amounts.

Repro with policyengine[uk]==4.11.0 (policyengine-uk@2.88.20):

from policyengine.tax_benefit_models.uk import uk_latest
for name in [
    "gov.dwp.pension_credit.guarantee_credit.minimum_guarantee.SINGLE",
    "gov.dwp.pension_credit.guarantee_credit.minimum_guarantee.COUPLE",
    "gov.dwp.pension_credit.guarantee_credit.carer.addition",
    "gov.dwp.pension_credit.guarantee_credit.severe_disability.addition",
]:
    for value in uk_latest.get_parameter(name).parameter_values:
        if value.start_date.year == 2026:
            print(name, value.value, value.start_date, value.end_date)

Actual 2026 values include:

• minimum_guarantee.SINGLE: 229.3929826081932
• minimum_guarantee.COUPLE: 350.12286608501375
• carer.addition: 47.94659726635978
• severe_disability.addition: 86.0813766865874

Published 2026-27 weekly rates are:

• Standard minimum guarantee, single: 238.00
• Standard minimum guarantee, couple: 363.25
• Severe disability addition: 86.05 for one qualifying adult, 172.10 where both qualify
• Carer addition: 48.15

Sources:

• GOV.UK Pension Credit page: https://www.gov.uk/pension-credit/what-youll-get
• GOV.UK Benefit and pension rates 2026 to 2027: https://www.gov.uk/government/publications/benefit-and-pension-rates-2026-to-2027/proposed-benefit-and-pension-rates-2026-to-2027
• SI 2026/148 PDF updates regulation 6(a) from £346.60/£227.10 to £363.25/£238.00: https://www.legislation.gov.uk/uksi/2026/148/pdfs/uksi_20260148_en.pdf

This surfaced while comparing Axiom RuleSpec UK against PolicyEngine over local Enhanced FRS. Positive-weight EFRS benunits imply weekly standard minimum guarantee values around 229.39 and 350.12, while the generated RuleSpec from the 2026 instrument returns 238.00 and 363.25.

Expected: update the Pension Credit guarantee parameters for 2026-27, with the correct April 2026 effective-date semantics.

Locate the parameter definitions exposed through uk_latest for the four named Pension Credit guarantee parameters, then inspect how 2026 values and effective dates are represented. Compare them with the published 2026-27 weekly rates and verify the April 2026 effective-date semantics using the provided reproduction; done means all listed rates and dates match the published sources.